In order for any electric circuit or device (a “load”) that runs on electricity to operate properly, electric power must be supplied to it; and this power must be supplied al levels of current and voltage, and within tolerances, that are specified for a particular load. A switched-mode power converter is a device that receives electric energy from a raw input source generally not suited for a load and supplies a regulated output of electric energy in a form and an amount that is suitable for a particular load.
A switched-mode power convener includes at least one primary switch that is used to switch a direct current input on and off to produce a time-varying voltage and current. The direct current input may be supplied by a battery or be derived by rectifying an alternating current. A switched-mode converter includes a magnetic storage element, which may be an inductor, and frequently is a transformer for electrically isolating input from output. The time-varying voltage and current is coupled to the magnetic storage element. When the DC input is switched on, a magnetic field is induced in the magnetic storage element, and when the DC input is switched off, the magnetic field collapses, transferring the stored energy into an electrical current.
A switched-mode power converter also includes a circuit for controlling the primary switch or switches used to switch the input current on and off. The time period when the switch is closed is referred to as TON or simply the “on” period and the time period when the switch is off is referred to as TOFF or simply the “off” period. A switching time period TS or “duty cycle” is defined as TS=TON+TOFF. The control circuit can regulate the amount of power that is delivered to the load by making the on period longer or shorter for a fixed duty cycle. Alternatively, the amount of power delivered can be regulated by making the duty cycle longer or shorter for a fixed on period. Because the output voltage of a switched-mode converter generally depends on the input voltage, the duty cycle, and the load current, a feedback signal indicative of the output voltage or current is provided to the control circuit for use in regulating the duty cycle.
In addition to the primary switch, a switched-mode power converter commonly includes one or more switches that control the flow of current and prevent it from flowing in certain directions within the converter circuit. These devices for controlling the flow of current in a switched-mode power converter are referred to herein as “secondary switches.”
Switch mode power converters in a variety of topologies are known. Some of the primary topologies that use an inductor as the magnetic storage element are the buck, boost, and buck-boost topologies. The buck topology is shown in FIG. 1, the boost topology is shown in FIG. 2, and the buck-boost topology is shown in FIG. 3. Other important topologies that use an inductor as the magnetic storage element are the full-bridge, half-bridge, and Cuk topology. In FIGS. 1-3, the reference numbers 20 and 22 identify, respectively, the primary and secondary switches. In other topologies, the primary and secondary switches are located similarly to the locations shown in FIGS. 1-3. In general, some of the principal switch mode power converter topologies that incorporate a transformer as a magnetic storage element to achieve electrical isolation include the flyback converter which is derived from the buck-boost converter, the forward converter which is derived from the buck converter, the forward-flyback converter, the push-pull converter which is derived from the buck converter, and the half-bridge and full-bridge converters which are derived from the buck converter. The forward converter, a modified version of the forward converter known as the two-transistor forward converter, the forward-flyback converter, and the half-bridge and the full-bridge converters have, in addition to a transformer, one or more inductors on the output or secondary side of the transformer dial serve as additional magnetic storage elements.
In some topologies, energy is transferred to the magnetic storage element (or elements) and to the load during the TON portion of the duty cycle. This may be referred to as a forward transfer. The transfer of energy from the magnetic storage element to the load that occurs during the TOFF portion of the duty cycle may be referred to as the flyback transfer. If the topology has two secondary switches and both switches conduct current during the flyback transfer, the topology is referred to as a symmetrical topology. On the other hand, if the topology has two secondary switches and one switch conducts only during the forward transfer and other conducts only during the flyback transfer, the topology is referred to as an asymmetrical topology. In addition, if the topology has only one secondary switch and the switch conducts during the forward and flyback transfer, the topology is considered an asymmetrical topology. Symmetrical topologies include the full-bridge, half-bridge, and push-pull. Asymmetrical topologies include the forward converter, the two-transistor forward converter, and the forward flyback converter with an active clamp. In addition, to the symmetrical and asymmetrical topologies, there is a third type of topology in which there is only one secondary switch and it conducts only during the flyback transfer. The flyback converter is an example of this third type of topology.
Diodes have traditionally been be used as a secondary switch within switched-mode power converters. A diode has input and output terminals and may be either forward- or reverse-biased. An “ideal” diode will conduct a current when the input terminal is at a higher voltage than the output by a “cut-in voltage,” that is, when it is forward-biased, and will not conduct when the voltage difference is below the cut-in voltage, that is, when it is reverse-biased. Thus, the diode acts as closed switch when it is forward-biased and as an open switch when it is reverse-biased.
An advantage of using a diode switch in a switched-mode power converter is its simplicity. However, diodes have a fixed forward voltage drop that is independent of the amount of current flowing in the diode. Typically, the voltage drop across a diode is on the order of 0.5 to 1 volt, or more. The electric power consumed by any device is the product of the voltage drop across it times the current through it. Because of the fixed forward voltage drop, high levels of power can be dissipated in the form of heat in a switched-mode power converter that uses ordinary diodes. Moreover, power losses increase significantly with the amount of current that flows in the diode. A special type of diode known as a Schottky diode has a smaller forward voltage drop than an ordinary diode, typically on the order of 0.15 to 0.45 volts. To achieve greater efficiency, Schottky diodes are commonly used as switches in switched-mode power converters.
Beginning with the invention of the transistor and later the integrated circuit, there has been and continues to be a trend to circuits with smaller and smaller features. In addition, there has been and continues to be a trend to circuits with higher and higher switching speeds. These trends are widely recognized. Within the field of power engineering, several related trends are recognized which concern the requirements of switched-mode power converters that supply power to these smaller and faster circuits. Specifically, switched-mode power converters must be made to fit within dimensions that are increasingly smaller, they must be able to supply voltages that are increasingly lower, and supply currents that are increasingly higher. For example, the typical required supply voltage has dropped from 5 volts to 3.3 volts, with devices in the near future expected to require only 1.5 volts. In addition, gigahertz switching speeds are now common and the current required in a circuit generally increases with frequency. Moreover, because many circuits today are battery powered so that devices incorporating them can be used with mobility, there is a need for switched-mode power converters that are ever more efficient. Yet another requirement of present electronic circuits stems from the power-saving “sleep mode” that many such circuits enter during periods of inactivity. A switched-mode power converter must be able to supply levels of energy for both normal and sleep modes. While Schottky diodes are more efficient as secondary switches than ordinary diodes, further improvements in efficiency and miniaturization would be desirable.
A synchronous rectifier is a type of switch that is significantly more efficient than a diode. In addition, a synchronous rectifier permits the use of smaller magnetic and electric storage elements. A synchronous rectifier is typically a MOSFET (metal-oxide-semiconductor field-effect transistor) and bypass diode embedded within the same silicon structure. In comparison to the 0.15 to 0.45 voltage drop of a Schottky diode, the voltage drop across a MOSFET during conduction is typically less than 0.1 volt. A MOSFET is a three terminal device in which a current will flow between a source terminal and a drain terminal if a control signal is applied to a gate terminal, provided of course that the source and drain are at different voltages. (The MOSFET is operated in cut-off and saturation modes, and accordingly when the control signal is applied it must be sufficient to place the MOSFET in saturation.) The MOSFET stops conducting and acts like an open circuit when the control signal is removed from the gate. Thus, the MOSFET acts as closed switch when there is no control signal on the gate and as an open switch when a control signal is applied.
MOSFET devices have properties that can create problems in certain switched-mode power converters or under certain conditions. Unlike a diode, a MOSFET is capable of conducting current in either direction, that is, from drain to source or from source to drain. If the load transitions from a slate where it needs a lot of power to one where it needs much less power, it is possible that current can flow within the converter circuit in reverse of the intended direction. Reverse current flow can mean that energy is transferred from the load to the input source, reducing converter efficiency. Thus, synchronous rectifiers have generally only been used if the direction of current flow can be guaranteed. Another potential problem arises from the fact that the primary switch in a switched-mode converter is commonly a power MOSFET. If the primary switch is turned on before the secondary switch has turned off, both MOSFETs will simultaneously conduct current, resulting in an undesirable “shoot-through” current. A shoot-through current generates large power losses and leads to rapid failure of the switching devices. For this reason, the control synchronous rectifiers must also be precisely regulated.
As mentioned, the use of a diode as a secondary switch in a switched-mode power converter has the advantage of simplicity: a diode does not require a control signal. When a synchronous rectifier replaces a diode, some method for providing a control signal is required. One method for providing a control signal is the “self-driven” method. In the self-driven method, the gate of the synchronous rectifier is directly coupled to a secondary winding of a transformer. However, in some circuit topologies, it may not be possible to implement the self-driven method. In addition, the self-driven method may not provide the required degree of precision. Self-driven topologies are typically not suited for applications where the input power or required load power fluctuates. Further, self-driven topologies are typically not suited for situations where two or more converters are coupled in parallel to supply a single load. In another method, the output voltage may be used as a feedback signal for a secondary switch control circuit. The use of output voltage suffers from the problem that it typically changes too slowly to provide rapid feedback concerning changes in the power requirements of the load. This slow response to fast current transients fails to protect switches and leads to devices failure. In yet another method, the output current may be used as a feedback signal for a secondary switch control circuit. While output current changes faster than output voltage, providing faster feedback than the voltage method, the use of the output current requires a current sensor. Thus, a disadvantage of use output current as a feedback signal is that an additional component is needed, a component which reduces efficiency by adding output impedance.
Accordingly, there is a need for a method and apparatus for providing a control signal for controlling a synchronous rectifier that does not suffer from the disadvantages of the known methods for providing a control signal.